To evaluate the safety and effectiveness of epigallocatechin-3-gallate (EGCG) solution treating the acute severe dermatitis in patients receiving radiotherapy. This phase I research enrolled patients with thoracic cancer receiving radiotherapy at Shandong Cancer Hospital and Institute in Shandong, China. EGCG solution was sprayed to the radiation field when grade III radiation-induced dermatitis (RID) first appearance. EGCG concentration escalated from 660 to 2574 μmol/L using modified-Fibonacci dose-escalation. RID and related symptoms were followed up every day. Between March 2021 and November 2021, 19 patients were enrolled in this phase I research. The median dose of grade III RID first observation was 44 Gy (30.6–52 Gy). As the EGCG treatment was performed continuously, all these grade III RID reactions were significantly decreased to grade I or grade II RID at three days after use of EGCG (p < 0.001). Significant relief can be observed in burning sensation (p < 0.001), tractive sensation (p < 0.001), tenderness (p < 0.001), erythema (p < 0.001), itching (p < 0.001) and pain (p < 0.001) after 15 days of EGCG treatment. No radiation therapy delay or interruption for all 19 patients. No adverse events were observed and reported associated with EGCG. The highest dose of this Phase I trial (2574 μmol/L) was recommended for continuous Phase II trial for further evaluation. In this phase I clinical research, use of EGCG solution is safe and can significantly relief grade III RID in patients receiving radiotherapy. Thus, EGCG might be a new choice for acute sever RID.Trial Registration: ClinicalTrials.gov Identifier: NCT02580279 (Full date of first registration: 12/2014).